Protection system for an image tube

ABSTRACT

A protective system for an image tube to prevent high intensity illumination of the image tube from damaging the high sensitivity electrodes within the image tube. The protection system consists of a magnetic field with associated control circuit responsive to the amplitude of the input radiation to deflect an electron beam generated by said input radiation from the high sensitivity electrode within the image tube during high intensity illumination of the image tube.

United States Patent 5 1191 111.1 3,864,594 Svensson 1 Feb. 4, 1975 1 PROTECTION SYSTEM FOR AN IMAGE 3,586,773 6/1971 Niemyer et a1. l78/D1G. 29

TUBE FOREIGN PATENTS OR APPLICATIONS Emil Svensson, Ellicott City, 1,179,765 5/1969 U.S.S.R. 178/7.2 E [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. Primary Examiner-Malcolm F. Hubler Assistant Examiner-N. Moskowitz [22] Flled' 1969 Attorney, Agent, or Firm-C. F. Renz [21] Appl. No.: 889,602

[57] ABSTRACT U-S- Cl.

A protective ystem for an image tube to prevent CL; intensity illumination of the image tube from damag- Field of Search 315/10; 178/7-2 1 ing the high sensitivity electrodes within the image tube. The protection system consists of a magnetic References cued field with associated control circuit responsive to the UNITED STATES PATENTS amplitude of the input radiation to deflect an electron 2,571,306 10/1951 Szegho l78/DlG. 29 beam generated by Said input radiation from the high 3,061,673 10/1962 Fathauer et a1. 178/792 sensitivity electrode. within the image tube during high 3,273,458 9/1966 Kohler..... 88/61 intensity illumination of the image tube. 3,377,427 4/1968 Fischer.... 178/72 E 3,471,640 10/1969 Tanner 178/7.2 E 6 Claims, 2 Drawlllt; Flgures SWITCHING CURRENT SOURCE DATENIEDFEB 41915 5864.594

SWITCHING CURRENT DEVICE SOURCE '1, \68 FIG. 1

WITNESSES \NVENTOR I Emil L. Svensson B? 52 Q/WM, paw ATTORNEY Z PROTECTION SYSTEM FOR AN IMAGE TUBE BACKGROUND OF THE INVENTION This invention relates to a protective system associated with an electron image tube for the protection of 5 the electron image tube from a high intensity light input.

It is well known in the art that certain television camera tubes and particularly those of the low light level type due to their high sensitivity have electrodes that are susceptible to damage when the tube is exposed to high intensity light input. In most of these camera tubes, the light is directed onto a photocathode which emits photoelectrons in response to the input radiation. A high potential electric field is provided to accelerate these photoelectrons either directly onto the target of an image tube or onto an intermediate amplification electrode. These high energy electrons may be of sufficientnumber due to high intensity light input to cause damage to the succeeding electrode member. The damage is normally a function of the intensity of the light and the duration of the high intensity radiation.

Several possible solutions to this problem have been suggested in the prior art. One suggested solution is of switching off the high voltage that accelerates the electrons from the photocathode to the succeeding electrode. This has not been found satisfactory in that the normal turn-off time of the high voltage supply is too slow to prevent damage to the tube. Another suggested solution is to provide suitable filtering means between the radiation source and the camera tube such that the amount of light directed onto the camera tube is substantially reduced in response to high intensity radiation directed onto the filter means.

SUMMARY OF THE INVENTION A protective system for an electron image tube is provided in which a magnetic field is provided about the electron image tube and in a position such as to provide a cross magnetic field of sufficient intensity to deflect the electron image generated by the photocathode completely off the succeeding electrode and thereby protect any succeeding electrode from high intensity radiation damage. The magnetic field is controlled by a light sensitive element which activates the magnetic field in response to a predetermined amount of radiation input.

The invention will become more readily apparent from the following exemplary description of the preferred embodiment in connection with the drawing.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic showing of an image system incorporating the teachings of this invention;

FIG. 2 is a diagrammatic view illustrating the operation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, an image system is shown comprised of an image intensifier tube 12 and a pickup tube 14. A fiber optic input window 15 is provided in the intensifier tube 12 having a photoemissive surface 16 responsive to input radiations. The window 15 is comprised of a plurality of glass fibers 13. The fibers 13 normally consist at least of a core portion of a high refractory index glass with a coating of low refractory index glass. The fibers 13 are bound together and bonded to provide the vacuum tight window 15. The image intensifier 12 also includes an output fiber optic window 18 having an output phosphor 20 located on the inner surface of the window 18. An electrical conductive coating 21 is normally provided on the surface of the output phosphor 20. The photocathode 16 may be operated at a potential of about a negative 15 kilovolts and the output phosphor 20 through the coating 21 is connected to ground potential. The electrons emitted from the photocathode 16 in response to input radiation are accelerated and directed onto the output phosphor 20 to give an intensified light image corresponding to the input radiation. Electrostatic focusing may be provided and suitable electrodes provided within the tube 12. The light output from the intensifier 12 is coupled to the camera tube 14.

The camera tube 14 may be of the SEC type described in US. Pat. No. 3,213,316 and includes an input fiber optic window 30 having a photocathode 32 provided on the inner surface. The photocathode 32 is operated at a potential of about a negative 8 kilovolts and the electron image emitted from the photocathode 32 is focused by electrostatic means onto a storage target 34 to provide a charge image. This charge image may be read out by means of an electron gun 36 illustrated as a cathode. An output signal is derived from the target 34 in response to read out by scanning the electron beam from the gun 36 over the target 34. The output signal may be connected to a suitable display device. The target structure 34 may operate at substantially ground potential. An insulating member 50 of fiber optic material may be provided between the output window 18 of the image intensifier 12 and the input window 30 of the camera tube 14.

A suitable lens system is provided in front of the input window 15 to project the input radiation image onto the plane of the photocathode 16.

A protective system 60 is positioned exterior of the image intensifier tube 12 and includes a pair of magnetic deflection coils 62 and 64 positioned about the image intensifier envelope and between the photocathode l6 and the output phosphor 20 and placed at about l80 from each other. The two deflection coils 62 and 64 may be connected in series to provide a magnetic field transverse to the path of electron beam between the photocathode l6 and the output phosphor 20. One terminal of the deflection coil 62 may be connected to ground while the other terminal of the deflection coil 64 is connected through a switching device 68 to a current source 72. The switching device 68 may be of any suitable type such as a switching transistor. A light sensitive device 70, such as a photocell, is positioned so as to be exposed to input radiations without interferring with the input to the tube 12. The light sensitive device 70 is responsive to the input radiation and generates a signal which is applied to the switching device 68 causing the switch 68 to close at a predetermined value of input radiation and applying the current source 72 to the coils 62 and 64. The application of this magnetic field causes the electron image generated by the photocathode 16 to be deflected in such a manner as to totally deflect the beam from the output phosphor 20. The path of the normal electron image is illustrated by the solid lines in FIG. 2 and the path of the deflected electron image indicated by the dashed lines.

With the magnetic field directed into the paper as illustrated in FIG. 2, the electron image will be directed downwardly off the output phosphor 20. In response to radiations directed onto the light sensitive device 70 of a value less than that initially selected, the protective circuit 60 is non-responsive and the magnetic field is not applied to the electron beam generated by the photocathode 16. However, if the light intensity exceeds the above value then the light sensitive device 70 closes the switching device 68 and current is applied to the coils 62 and 64. The resulitng magnetic field deflects the electron beam from the photocathode 16 completely off the output target 20. On removal of high intensity light, the switch 68 will open.

lt has been found that by providing a low inductance type circuit the response of the system to high input radiations can be as fast as 100 microseconds. It is found that this is adequate to protect the image system from destruction and particularly the target 34 in the SEC pickup tube 14.

I claim:

1. An image system comprising an image tube, said image system responsive to input radiations, said image tube comprising means for generating an electron beam and directing it onto a target, deflection means positioned exterior to said image tube for deflecting said electron beam, means responsive to said input radiations greater than a predetermined value to cause energization of said deflection means to deflect said electron beam from the active area of said target.

2. The system in claim 1 in which said target may suffer permanent damage from input radiations above said predetermined value of input radiation.

3. The system in claim 1 in which said deflection means comprising magnetic means.

4. The system in claim 1 in which said deflection means comprises a plurality of deflection coils.

5. The system in claim 1 in which said means responsive to said input radiation comprises a light sensitive device positioned exterior of said envelope to intercept said input radiations and responsive thereto, and switching means responsive to the output of said light sensitive device to apply excitation to said deflection means.

6. The system in claim 1 in which said means responsive to said input radiations and said deflection means is responsive to deflect said electron beam in about microseconds. 

1. An image system comprising an image tube, said image system responsive to input radiations, said image tube comprising means for generating an electron beam and directing it onto a target, deflection means positioned exterior to said image tube for deflecting said electron beam, means responsive to said input radiations greater than a predetermined value to cause energization of said deflection means to deflect said electron beam from the active area of said target.
 2. The system in claim 1 in which said target may suffer permanent damage from input radiations above said predetermined value of input radiation.
 3. The system in claim 1 in which said deflection means comprising magnetic means.
 4. The system in claim 1 in which said deflection means comprises a plurality of deflection coils.
 5. The system in claim 1 in which said means responsive to said input radiation comprises a light sensitive device positioned exterior of said envelope to intercept said input radiations and responsive thereto, and switching means responsive to the output of said light sensitive device to apply excitation to said deflection means.
 6. The system in claim 1 in which said means responsive to said input radiations and said deflection means is responsive to deflect said electron beam in about 100 microseconds. 